My experience is that tests model the expected behaviour and that type systems cannot. Therefore I get lots of value out of tests where types are an impediment.

My experience is that tests model the expected behaviour and that type systems cannot. Therefore I get lots of value out of tests where types are an impediment. Static typing from the beginning just sets me into a big design up front problem where I try to anticipate the flexibility that I will need and I start building lots of boilerplate abstractions that end up never being used or restricting me later. I find it useful to write imperfect code with only the constraint that it is testable and then revise the structure and any types I need as it becomes evident that they are valuable in the context.